MATLAB: Use for-loop on characters in a textfile

encryptionforletterloopMATLABnumberto

I have a plain text file that is going to be encrypted – where I'll use double as part of the process. I read the text file
text = fileread('textfile.txt')
Then I need to get the letters into numbers
lettertoNumber = double(text)
Now comes the part I'm struggling with. I need to have a for-loop that goes through lettertoNumber, which also needs to take lower/captial letters into consideration.
for ii = 1:length(lettertoNumber)
    ?? = ???????
    ?? = ???????
end
Greatly appreciate any input.

Best Answer

Use lookup tables. Like
MAP_INVALIDS_TO = -1;
mapping = zeros(1,255) + MAP_INVALIDS_TO;
mapping('a':'z') = 1:26;
mapping('A':'Z') = 1:26;
mapped = mapping(text);      %valid to use characters as indices
mapped(mapped == MAP_INVALIDS_TO) = [];      %delete characters that had no mapping
Modified versions of this can be used for situations such as encoding the other characters as pairs of values, or for using "shift-in" / "shift-out" to switch between encodings.
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